Wow - this sounds like another great update to an already great mod! Special thanks to the individual creature makers.
Just wondering, should I now make Creatures an ESM? It's become quite an overhaul of MW's creature/list system and it may help resolve conflicts. Can anyone more experienced them me shed any light on this?
In my tests I haven't found esms to behave any different from esps, except that they load earlier, which might be undesirable for a mod that should not have its objects merged. (FWIW, I put Creatures into merged_objects and see no problems at all, but anyway - the current standing recommendation is to not include it, and that means that it's probably better if Creatures loads pretty late, so turning it into an esm would be counter-productive).
In Pre-MCP times, one of the incentives for turning a mod into an esm was that esm didn't cause (as much) doubling. The reason for this was that Morowind was often able to re-match esms correctly when the load order had changed, but not esps. The Morrowind Code Patch fixes this, so esms have no advantage over esps in that regard.
I've also seen many claims that "esms cannot change esms", but my tests indicate otherwise.
Personally, I suspect that the Morrowind engine, after having been fixed with the MCP, handles esms and esps almost identically. If that's correct, then the only advantage of turning a mod into an esm would be that other mods can be made dependent on it in the construction set, so that they can change references added by your mod without doubling them. And even that is perhaps just a limit of the construction set, not of the game engine (it may be possible to achieve the same effect by adding an esp as another mod's master, but I haven't tested this).
So, in short: If your mod adds many new *references* (not objects) that other mods may want to modify, then turning it ointo an esm might be a good idea. Otherwise it might be counter-productive since it would cause Creatures to load much earlier, making it more susceptible for overrides.